U.S. Pat. No. 4,921,767, issued to Datta et al. on May 1, 1990, describes a method of electrophotographically manufacturing a luminescent screen assembly for a CRT using triboelectrically-charged matrix and phosphor materials. In the patented method, a photoconductive layer, overlying a conductive layer, is electrostatically charged to a positive voltage and exposed, through a shadow mask, to light from a xenon flash lamp, located in a lighthouse. The exposure is repeated a total of three times, from three different lamp positions, to discharge the areas of the photoconductive layer where the light-emitting phosphors subsequently will be deposited to form the screen. The shadow mask is removed, and triboelectrically-(negatively)charged particles of light-absorptive matrix material are deposited on the positively-charged areas of the photoconductive layer. After the matrix is formed, the photoconductor is recharged to a positive voltage and then exposed to light through the shadow mask, to discharge the areas where the first of three triboelectrically-(positively)charged, light-emitting phosphors will be deposited. Prior to phosphor deposition, the shadow mask, again, is removed from the faceplate panel. Then, the first triboelectrically-(positively)charged phosphor is deposited, by reversal development, on the discharged areas of the photoconductive layer. The process is repeated twice more to deposit the second and third color-emitting phosphor materials.
A drawback of the patented method is the difficulty of obtaining sufficient opacity in the deposited matrix. The opacity is proportional to the amount of light-absorptive material that is deposited in the matrix lines. In the electrophotographic screening process, a high opacity matrix requires a high voltage contrast in the patterned electrostatic image formed on the photoconductive layer. In a 51 cm diagonal tube the matrix lines are only about 0.1 to 0.15 mm (4 to 6 mils) wide and have a pitch, or spacing, between adjacent matrix lines of only about 0.28 mm (11 mils), compared to a width of about 0.27 mm and a pitch of about 0.84 mm (33 mils) for phosphor lines of the same emissive color, thus, the reduced line size and spacing of the matrix lines increase the difficulty of forming images in the lighthouse. The combined effects of the extended width of the flash lamp and the diffraction of the light passing through the slots, or apertures, in the shadow mask, for the three exposures required for the matrix image pattern, produce overlapping penumbras on the photoconductive layer that are not totally black, but which have a light level of about 25% of that found in the highly illuminated areas of the layer. In other words, the exposure through the shadow mask does not produce a light pattern that is either totally illuminated or totally black, but instead produces a pattern of light areas separated by gray penumbras of reduced light intensity. Accordingly, the voltage contrast of the electrostatic image is much lower for the matrix exposure than for the phosphor exposures, and the resultant matrix lines are less opaque than desired, especially at the edges of the lines. It has been determined that because of the above-described light diffraction pattern through the shadow mask, it is not possible to improve the voltage contrast by increasing the exposure time, since the voltage contrast of the photoconductive layer reaches a maximum and then decreases with an increase in light exposure time.